#define _CRT_SECURE_NO_WARNINGS 1
#include<string>
#include<iostream>
using namespace std;
 struct TreeNode {
     int val;
     TreeNode *left;
     TreeNode *right;
     TreeNode() : val(0), left(nullptr), right(nullptr) {}
     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 };
 
class Solution {
public:
    void _tree2str(TreeNode* root, string& s)
    {
        if (root == nullptr)
        {
            return;
        }
        s += to_string(root->val);
        if (root->left != nullptr || root->right != nullptr)
        {
            s += '(';
            _tree2str(root->left, s);
            s += ')';
            if (root->right != nullptr)
            {
                s += '(';
                _tree2str(root->right, s);
                s += ')';
            }
        }
    }
    string tree2str(TreeNode* root)
    {
        string s;
        _tree2str(root, s);
        return s;
    }
};